Why don't clouds go all the way to the ground?

[IO540]

We seem to get overcasts down to 200ft or even 100ft, but almost never below that, over a large area of flat ground.

Unless it's fog but that's caused by something different.

Something happens to the temp and/or the dewpoint, close to the ground. Does the proximity of the ground warm the air, or does it dry it out a bit?

Does anybody know?

[SkyHawk-N]

We seem to get overcasts down to 200ft or even 100ft, but almost never below that, over a large area of flat ground.
Unless it's fog but that's caused by something different.
Something happens to the temp and/or the dewpoint, close to the ground. Does the proximity of the ground warm the air, or does it dry it out a bit?
Does anybody know?

A stratus cloud at ground level is commonly called fog.

[Deano777]

It's probably due to the fact that the heating of the atmosphere is done by radiation, the sun heats the ground and the ground then heats the surrounding air, so it will always be warmer closer to the ground, (unless we have an inversion but that's irrelevant), I was driving home from work yesterday and noticed that the stratus clouds couldn't have been no more than 100ft above the surface, I was going to check Form 214 to see what the base was like but forgot.
Yes stratus to ground level will be called fog, generally though this only forms early morning and mid evening when the temperature falls to dewpoint as we get closer to winter, and has to be cold enough to last all day as we all know, but I would imagine that during this time of year the sun is strong enough to warm the ground sufficiently to keep the temperature away from the dewpoint even with 8/8 coverage.

Dean

[Whopity]

Why don't clouds go all the way to the ground?
Clouds form because moist air is forced to rise; they are going up rather than coming down! The five causes are:
Orographic
Turbulence
Convergence
Frontal
Convection

It takes time for the air to rise and cool, so there will invariably be a space before the cloud forms.

[ChrisVJ]

I live in the mountains of BC and frequently comment to Mrs VJ that it is not like living in London. The clouds drift through here and often reach down to ground level. Not fog but distinct wispy trails. Weird.

[Deano777]

It takes time for the air to rise and cool, so there will invariably be a space before the cloud forms.

But not always, if the surrounding environment is cold enough for the airmass to reach dewpoint then that mass will condense no matter how low or high the moist air gets to, I would say moisture condensing is a direct result of temperature and an indirect result of height because we all know about lapse rates don't we? (again, take inversions out the equation)

[IO540]

OK keep guessing :)

I don't think the reson for the usual 100ft+ gap near the ground is much to do with the normal lapse rate.

Apart from anything else, any pilot with a working OAT gauge knows the lapse rate is meaningless a lot of the time.

The proximity of the ground must be the cause. I just wonder what the mechanism is.

[Whopity]

I would say moisture condensing is a direct result of temperature and an indirect result of height

Exactly, but in most cases i.e. when there is no fog, the cloud is formed by rising air.

The effects of the boundary layer i.e. the air directly in contact with, and affected by the Earth's surface are seldom mentioned in any aviation met text books, but were of great interest to the scientists at the Met Office. Kinetic heating is one of the factors.

[Deano777]

OK keep guessing :)

I don't think the reson for the usual 100ft+ gap near the ground is much to do with the normal lapse rate.

Apart from anything else, any pilot with a working OAT gauge knows the lapse rate is meaningless a lot of the time.

The proximity of the ground must be the cause. I just wonder what the mechanism is.

IO

It must be to do with what I stated in my first post surely, that is, because the temperature of the atmosphere is governed by surface heating which in turn heats the air in contact with it, so the hotest place in any airmass would surely be at the surface, and this would then transfer up due to mixing and convection (no matter how small) to raise the temperature a tad in air that is 0ft - 100ft - 200ft+ above, so it would have to be either pretty cold at the surface, or a serious amount of moisture present

thoughts?

[Deano777]

Exactly, but in most cases i.e. when there is no fog, the cloud is formed by rising air.

The effects of the boundary layer i.e. the air directly in contact with, and affected by the Earth's surface are seldom mentioned in any aviation met text books, but were of great interest to the scientists at the Met Office. Kinetic heating is one of the factors.

Totally agreed

[theresalwaysone]

OH DEAR looks like quite a few of you need some PPL MET revision!

My comments are in bold type

[quote=Deano777]It's probably due to the fact that the heating of the atmosphere is done by radiation, the sun heats the ground and the ground then heats the surrounding air, so it will always be warmer closer to the ground, (unless we have an inversion but that's irrelevant), /quote]

Actually its not irrelevant at all, in fact its the opposite, a contributory factor in radiation fog is the inversion caused by high pressure which effectively puts the lid on the fog layer. Inversions and isothermal layers also determine the cloud tops and the extent of the vertical depth of the fog layer and therefore its persistence and cloud base. Radiation itself also contributes to inversions by modifying the lapse rate by condensation. e.g. due to the release of latent heat in the lower levels. Don’t think in terms of the base lifting, think in terms of the base evaporating because that’s generally what happens when the base is lifted by re-radiation. But see below because the base can also be lifted by geostrophic mixing

We seem to get overcasts down to 200ft or even 100ft, but almost never below that, over a large area of flat ground.

Not quite true, different areas have different fog records -- Leeds Bradford is particularly prone to frontal and radiation fog. Aberdeen, Teeside, Newcastle, Edinburgh, Plymouth, Guernsey, Alderney are particularly prone to seasonal advection fog. With radiation fog some of the densest occurs when the sun has been below the horizon the longest so that this occurs around 3- 7 in the morning. A lot of people wake up not even knowing there has been any fog! But get a job flying night freight and you may have a different opinion of the number of evenings and mornings with cloud below 200ft!
Unless it's fog but that's caused by something different.
Cloud and fog are the same, visible water droplets and the bottom line cause is the same--simply put, fog is a cloud on the ground or just above it and fog is defined by visibility.


Something happens to the temp and/or the dewpoint, close to the ground. Does the proximity of the ground warm the air, or does it dry it out a bit?

That’s a very wide reaching statement-- the earth certainly warms the air but for 'drying it out a bit' substitute, 'modifies the relative humidity and dew point' When air condenses it releases latent heat and this can modify the cloud base. There are many variable factors depending on the degree of radiation the and the amount of moisture. EG for instance you will see fog sometimes over the airfield grass area but not in the terminal area.


A stratus cloud at ground level is commonly called fog.

Not correct-- fog is reduction in visibility caused by visible water droplets in the atmosphere. nothing to do with cloud type. Any low to medium cloud can produce fog but it is more normally associated with low stable stratiform cloud Orographic Cumulus cloud can produce fog, climb a mountain with is hat on and see!





Yes stratus to ground level will be called fog,
Not correct-- again fog is defined by visibility not cloud type or height.
A cloud producing 900 metres vis is fog and the same cloud producing 1100 metres is mist for aviation purposes

generally though this only forms early morning and mid evening when the temperature falls to dewpoint as we get closer to winter, and has to be cold enough to last all day as we all know, but I would imagine that during this time of year the sun is strong enough to warm the ground sufficiently to keep the temperature away from the dewpoint even with 8/8 coverage.

Again technically incorrect. Generally radiation fog forms in the evening and is burnt off in the early morning. Evaporation of the lower layer due to incoming solar radiation depends on depth of low cloud/fog layer, amount of time sun is above the horizon(most persistent and deepest radiation fogs being in Nov, Dec Jan when the sun is above the horizon for the shortest time) and whether any upper layer of cloud forms above fog layer a factor very often missed. The strength of the sun is not the answer, unless you know something that scientists don’t, its strength does not vary but it’s the time above the horizon that the sun radiates the earth that is the clue



Quote:
Clouds form because moist air is forced to rise; they are going up rather than coming down! The five causes are:
Orographic
Turbulence
Convergence
Frontal
Convection

Not comprehensive--- cloud/fog can form by downward motion of air KATABATIC and horizontal movement of air ,ADVECTION. If you fly in the South West, the Channel Islands or the East Coast you will know all about Advection fog. Katabatic local effects cause downslope fog at some UK airports


Quote:
Originally Posted by Whopity
It takes time for the air to rise and cool, so there will invariably be a space before the cloud forms.

Its pretty but little to do with the time

But not always, if the surrounding environment is cold enough for the airmass to reach dewpoint then that mass will condense no matter how low or high the moist air gets to,
I would say moisture condensing is a direct result of temperature and an indirect result of height because we all know about lapse rates don't we? (again, take inversions out the equation)

Condensation is a direct result of a parcel air being cooled to its dewpoint this can occur through the air being lifted to a cooler temperature or being cooled or warmed without lifting due to radiation or having its pressure changed.

There are three main reasons that low cloud can produce fog conditions in the UK.
I am referring to the cloud base as the condensation level

1. Radiation fog--zero wind will produce a ground level condensation level known as dew--5 kts mixing will lift the condensation just above the ground = fog --10 kts geostrophic mixing will lift the condensation level to produce low stratus as a broad brush rule --I dont think any of you mentioned wind really and it is important consideration in fog/ stratus cloud bases and is after all very basic PPL Met. Condensation level can also be modified by condensation itself, no two cases are the same, a large amount of condensation will produce a release of latent heat sufficient to change the condensation level. Re- radiation from the sun will modify the condensation level

2. Frontal Fog, mainly associated with the warm front or warm occluded front or warm sector weather or Cols,

3. Advection fog due to horizontal movement and famous for the North Sea Haar and Channel Islands and SW approaches fog or coastal fog-- these fogs can be very persistent and last for days--ive sat in Jersey for 3 days once waiting for TO RVR! Also can come from East as a matter of interest in Jersey.
There are many contributory factors to the condensation level or cloud base in ALL of these cases
The amount of moisture in the parcel of air over the airfield—this can be brought about by a far reaching homogeneous air mass such as very moist stable south westerly’s in the UK (excluding RPM) or local effect such as grass, nearby lake, the sea, or a heavy due .
The stability of the air mass—the more stable the air the more it wants to flop onto the ground the more unstable the more it is just looking for a trigger to rise.
The topography, anabatic and katabatic effects for some airfields are known for their fog pockets.
Mechanical turbulence-- certain directions over some airfields produce more up draughts again due to local topography or even man made objects
Heat, some areas convect or radiate more heat than others eg grass, tarmac, water
Condensation itself produces latent heat, which modifies the condensation level.
Rain from cloud lowers the dew point and temperature and again can modify the condensation level.



OK keep guessing file:///C:/DOCUME%7E1/JON/LOCALS%7E1/Temp/msoclip1/01/clip_image001.gif

I don't think the reson for the usual 100ft+ gap near the ground is much to do with the normal lapse rate.

In a lot of cases the cloud is on the ground but the visibility within that cloud is above 1000 metres and it’s classified as mist! At this stage it becomes irrelevant as the only aid to get you in now is a precision app and it’s going to be visibility limited at this altitude.

Apart from anything else, any pilot with a working OAT gauge knows the lapse rate is meaningless a lot of the time.

The proximity of the ground must be the cause. I just wonder what the mechanism is.

Well you are certainly guessing and your comment about normal lapse rates and OAT gauges shows you do not really understand the importance of lapse rates on cloud formation, cloudbase, vertical extent and tops, not, as you say, a lot of the time but all of the time. That’s why the MET office produce Tephigrams every day.


Quote:
I would say moisture condensing is a direct result of temperature and an indirect result of height

Exactly, but in most cases i.e. when there is no fog, the cloud is formed by rising air.
As already explained not totally correct

The effects of the boundary layer i.e. the air directly in contact with, and affected by the Earth's surface are seldom mentioned in any aviation met text books, but were of great interest to the scientists at the Met Office. Kinetic heating is one of the factors.

GREAT STUFF BUT PPL MET IS BASED ON SIMPLE THEORY, TRYING TO UNDERSTAND THE BASICS FIRST IS THE FIRST STEP. Leave the unnecessary complications to the scientists

[SkyHawk-N]

A stratus cloud at ground level is commonly called fog.
Not correct-- fog is reduction in visibility caused by visible water droplets in the atmosphere. nothing to do with cloud type. Any low to medium cloud can produce fog but it is more normally associated with low stable stratiform cloud Orographic Cumulus cloud can produce fog, climb a mountain with is hat on and see!

By this statement what you are saying is when a stratus cloud is at ground level it is NOT commonly called fog?

[IO540]

theresalwaysone

As it happens I do know what a tephigram or a skew-t is (and use them for flight planning) but it is laughable that you write such a pontificating piece first not knowing this is far divorced from not only PPL met syllabus but from any practical VFR or IFR met understanding requirement. It also barely answers my original question. Finally, a tephigram is hardly relevant that close to the ground; the distance I refer to is barely resolvable on the diagram and is not a lot more than the length of the piece of string the baloon is initially tied to...

This needs somebody with a PhD in physics. I did physics to somewhere part-BSc level but we didn't do atmospheric physics...

skyhawk

I don't think fog and stratus cloud are related, other than both having 100% RH. Fog is a local thing, but stratus can form over vast areas. It was stratus I was referring to in my original Q - why does it (almost) never go all the way to the ground.

The answer could be that the humidity falls near the ground. I don't think the temperature rises near the ground, to the very specific extent required for this to work the way it (almost) universally does, regardless of the surface covering, vegetation, etc.

[SkyHawk-N]

Just looking at the Met. Office web site

http://www.metoffice.gov.uk/education/secondary/students/clouds.html

quote: "Sometimes, when a sheet of stratus is affecting an area, the cloud base will be right down to the ground and will cause fog."

[SkyHawk-N]

Ah Google!

From NOAA (National Oceanic and Atmospheric Administrations)

Stratus
A low, generally gray cloud layer with a fairly uniform base. Stratus may appear in the form of ragged patches, but otherwise does not exhibit individual cloud elements as do cumulus and stratocumulus clouds. Fog usually is a surface-based form of stratus.

[Deano777]

Actually its not irrelevant at all, in fact its the opposite

Yes it is irrelevant, it was irrelevant to the question being asked and by what I was trying to say, the inversion layer would have been a totally different discussion, IO was asking why "In General" does the stratus cloudbase not go right to the surface.

A stratus cloud at ground level is commonly called fog.

Not correct-- fog is reduction in visibility caused by visible water droplets in the atmosphere. nothing to do with cloud type

I think you are being a little over pedantic here, if you are in 8/8 stratus cloud on the surface then you will "generally" be in fog & not mist.

Yes stratus to ground level will be called fog,
Not correct-- again fog is defined by visibility not cloud type or height.
A cloud producing 900 metres vis is fog and the same cloud producing 1100 metres is mist for aviation purposes

Again, we all know this, (although we were taught that fog is anything under 1000m vis & mist is anything over 1000m vis) but there is no way you can tell me that if a thick layer of stratus cloud was at the surface that it would be anything else other than fog, so again, you're being rather over pedantic.

The strength of the sun is not the answer, unless you know something that scientists don’t, its strength does not vary but it’s the time above the horizon that the sun radiates the earth that is the clue

:rolleyes: :rolleyes: , you think? thanks for the enlightenment, I'm sure us halfwits who frequent this forum did not know this, you have made my day.

Condensation is a direct result of a parcel air being cooled to its dewpoint this can occur through the air being lifted to a cooler temperature or being cooled or warmed without lifting due to radiation or having its pressure changed.

So what I said was right, even though height affects what happens with moisture within an airmass the point still remains, it's the temperature that is the over riding factors.

[SkyHawk-N]

I think you are being a little over pedantic here

Pedantic, that was the word I was looking for, thanks Deano777. There's always one! :rolleyes: ..... :}

[Deano777]

No worries :) and yes you are right, there is always one :rolleyes:

[bookworm]

I think we can all accept that condensation close to the ground is termed fog or mist, and that condensation with unsaturated air below it is termed cloud. The key issue is whether there is a process difference that would make fog in some way different from surface-level cloud.
Clearly hill fog is simply cloud that had the surface rise to meet it. ChrisVJ's observation is important, and concurs with mine. It's not really of the same nature as fog over flat areas.
The key seems to be the discontinuity in temperature at surface level. It is the surface that is heated by insolation or cooled by radiation, and changes in temperature are therefore significantly more rapid than in the neighbouring layer of air. Thus you would expect qualitative differences in condensation mechanisms at surface level.
In very simple terms, during daytime, the surface tends to be warmer than the air above it. If the moisture in the air arrived with it, this means that the surface is bound to be above the dewpoint of the immediately neighbouring air. Thus with any turbulence, there must be a mixing layer of unsaturated air below the condensation level. This tends to make the layer close to the surface less prone to cloud. There are, of course, ways in which this can break down, particularly if the moisture comes from rain falling into the air below.
At night, there is no insolation, and the surface tends to be cooler than its neighbouring layer. That means that the neighbouring layer will be prone to condensation if its dewpoint is high enough, while the air above remains warmer. Thus a layer of fog forms without a gap below it.

[bmoorhouse]

The strength of the sun is not the answer, unless you know something that scientists don’t, its strength does not vary but it’s the time above the horizon that the sun radiates the earth that is the clue
The price of pedantry is eternal vigilance - if you are going to lecture us as though we are dimwits then AT LEAST GET IT RIGHT. The phrase "strength of the sun" pretty obvioulsy refers to the PERCEIVED effect of the sun on the earth (i.e. received radiation for a fixed area over a fixed period of time). This is NOT just dependant on the time the sun is above the horizon but also the incident angle of the sun's rays, which depend on latitude and time of day.

The perceived strength of the sun (i.e.received radiation) is, I think you will find, a little lower at the north pole than at the equator due to the decreased angle of incidence, there is also the secondary effect of greater absorption for low incident light due to the greater thickness of atmosphre to be penetrated .

This may explain why there is ice at the poles and palm trees on the equator - but hey what do I know compared to "theresalwaysone"?